1. Field of the Invention
The present invention relates to a method of treating a surface of a mold, form or die (in the text, called “mold” for both terms) and to a mold having a surface treated by said method, and more specifically, it relates to a method of treating a surface of a mold which can improve the demoldability of molded products from the mold and to a mold having a surface treated by this method to achieve excellent demoldability.
2. Description of the Related Art
In case that a mold used for molding metal or resin has rough irregularities on its molding surface, such irregularities are transferred to a molded product (workpiece). Thus, finishing of the surface of the workpiece is required after molding. In addition, because the surface of the workpiece is engaged with the irregularities on the surface of the mold, which decreases the demoldability, the productivity significantly decreases during molding.
Therefore, typically, surfaces of molds are polished to a mirror finish by hand. This not only enables works to be finished in smooth surfaces, but also provides the required demoldability for works.
However, in recent years, the number of molds having a complex shape has been increasing, and demands for quick delivery of molds are rising. Thus, mirror-polishing of surfaces of molds by hand, which consumes a lot of time and effort, is an obstacle to meeting such demands and causes an increase in fabrication cost of the molds.
Moreover, depending on the shape and material of workpieces to be molded, the required demoldability cannot always be obtained even by mirror-polishing of surfaces of molds.
Thus, methods that can be substituted for mirror-polishing by hand have been proposed to improve the demoldability of works. Examples of such methods include increasing the draft angle of a cavity of a mold and subjecting the surface of the mold to treatment for improving slip, e.g., forming a diamond-like carbon film (DLC).
In contrast to applying a mirror finish to a surface of a mold, methods in which irregularities having a predetermined shape are formed on a surface of a mold have been proposed. An example of such methods is ejecting spherical abrasive particles at high speed onto the surface of the mold for molding rubber products to create spherical dimples with a diameter of from 10 μm to 30 μm, and then forming a hard chromium coating thereon (see claim 1 of the Publication of Japanese Patent No. 3154033).
The reasons for applying a mirror finish to a surface of a mold as described above are that, irregularities occurring on the surface of the mold are transferred to the surface of a workpiece, forming unwanted irregularities on the surface of the workpiece, and that the workpiece cannot be removed from the mold (for example, the workpiece cannot be moved parallel to the surface of the cavity of the mold) if the irregularities on the surface of the mold are engaged with the irregularities on the surface of the workpiece transferred from the mold.
On the other hand, excellent demoldability cannot always be obtained by mirror-finished surfaces of molds. According to one report, the demoldability can be improved by forming relatively fine recesses in the surface of the mold in the case of molding rubber products for cars (see the Publication of Japanese Patent No. 3154033).
However, when recesses are formed by blasting as described in the aforementioned Publication of Japanese Patent No. 3154033, not only are recesses formed in the surface of the mold by the collision of spherical abrasive particles, but also protrusions appear on the surface of the mold as a result of the base material of the mold being pushed out by the collision of the abrasive particles (see the enlarged view in FIG. 1B).
The protrusions generated in this way makes inroads into the surface of the workpiece produced by molding and thus interfere with the workpiece when the workpiece is removed from the mold, which decreases the demoldability.
Furthermore, when recesses are formed by the above-described method, the sharp protrusions generated simultaneously with the recesses serve as contact portions with the surface of the workpiece which make sliding contact with the surface of the mold when the workpiece is removed from the mold. Thus, the surface of the mold comes into point contact with the workpiece, leaving scratches on the surface of the workpiece when the workpiece is removed from the mold. Furthermore, because the load is concentrated on the peaks of the protrusions, the surface of the mold is easily worn.
Accordingly, an object of the present invention is to provide advantages in that improved demoldability achieved by creating irregularities are further enhanced by creating, on the mold surface, irregularities with the peaks being evened out and flattened by a blasting, that the surface of the workpiece is prevented from being scratched, and that wear of the mold is prevented by avoiding load concentration on one part of the surface of the mold with relative ease and without the need for laborious work such as polishing by hand.